Factor VIII is a large glycoprotein with which antihemophilic factor (procoagulant) and von Willebrand factor activities are associated. The separation of the procoagulant activity from the von Willebrand activity by a variety of methods, particularly gel filtration and sucrose density ultracentrifugation in high ionic strength buffers or by treatment with thrombin has led to considerable controversy about the molecular structure of factor VIII. To help resolve this controversy, factor VIII will be purified to homogeneity. The proteins associted with the two activities before and after thrombin activation will be separated by chromatography or sucrose gradient ultracentrifugation in high ionic strength buffers and chracterized by a variety of biochemical and immunological techniques. The reaction of thrombin with factor VIII will be studied in detail and, finally, the binding of calcium ions and phospholipids to factor VIII will be examined in purified systems by spectroscopic methods and biological assays. These studies should elucidate the relationship between the molecular structure of the factor VIII/von Willebrand factor protein(s) and its biological functions.